One of the high-temperature secondary battery systems being investigated as a power source for electric vehicles is the sodium-sulfur battery. These batteries offer a high specific energy and high specific power, both of which are required for electric vehicles. These battery systems may also be potential energy storage devices for electric utilities, where long life and low cost are more important than high specific energy and high specific power.
In a sodium-sulfur cell, a liquid anode of metallic sodium and liquid cathode of sulfur or sodium polysulfide are separated by a polycrystalline ceramic electrolyte of either sodium .beta.- or .beta."-Al.sub.2 O.sub.3. The operating temperature is typically between 300 and 400.degree. C. In this battery, sodium ions diffuse during discharge from the anode to the cathode by ionic conduction through the ceramic electrolyte. Usually the electrolyte is in the form of a tube with the liquid sodium anode in the interior of the tube. For high operating efficiency and low battery cost, it is essential that the conductivity of the electrolyte be as high as possible. For this reason, the preferred electrolyte is the sodium .beta."- Al.sub.2 O.sub.3, because of its higher ionic conductivity.
The ceramic electrolyte is usually manufactured by mixing powders of Al.sub.2 O.sub.3 and Na.sub.2 O (along with either Li.sub.2 O and/or MgO for stabilizing the .beta."- phase) in appropriate proportions forming a powder compact and subjecting the powder compact to any one of various sintering processes. Examples of these processes are disclosed in "Sintering Processes and Heat Treatment Schedules for Conductive, Lithia-Stabilized .beta."-Al.sub.2 O.sub.3 " by G. E. Youngblood, A. V. Virkar, W. R. Cannon, and R. S. Gordon: Bull Am. Ceram. Soc., 1977, 56. 206, and "Materials for Advanced High Temperature Secondary Batteries" by J. E. Battles: International Materials Reviews., 34. 1. In most sintering processes the formation of a grain boundary liquid phase is essential. The liquid phase formation can enhance the sintering kinetics, but results in a large grain size, which decreases the mechanical strength of the electrolyte. The liquid phase sintering also generates a residual NaAlO.sub.2 phase along the grain boundaries. The phase can react with moisture and further reduce the mechanical integrity of the ceramic electrolyte.
In U.S. Pat. No. 5,415,127 to Nicholson et al. and in "Formation and characterization of Na-.beta."-alumina single crystal films" by Aichun Tan, Chu Kun Kuo and Patrick S. Nicholson: Solid State Ionics 1993, 67. 131 is disclosed a method for the formation of Na-.beta."-Al.sub.2 O.sub.3 single crystal films. These films, because of their optical properties, have potential in solid-state lasers, holography, signal and image processing, phosphor chemistry, and other optical devices. The process comprises providing a single crystal substrate of alpha-alumina with an optically smooth surface parallel to a (001) crystal plane, and heating the substrate in the presence of a vapor containing Na.sub.2 O to react with the alumina and a stabilizing ion, such as lithium. The polished surface is required to form a single crystal. Otherwise a polycrystalline material is formed. The alumina is converted to Na-.beta."-Al.sub.2 O.sub.3 as it reacts with sodium oxide from the vapor. After conversion to Na-.beta."-Al.sub.2 O.sub.3 on the surface, further conversion requires that the Na.sub.2 O in the form of ions be transported through the already formed Na-.beta."-Al.sub.2 O.sub.3 and react with the alumina at a reaction interface. This provides a slow moving reaction front moving through the substrate. The kinetics of this process is rather sluggish. Disclosed is conversion of a 40 nm thickness at 1600.degree. C. in one hour. To convert a 0.5 mm thick electrolyte plate or tube for a battery electrolyte, several hundred hours at .gtoreq.1450.degree. C. would be required. This long time of formation materially adds processing and equipment costs to the fabrication. The limiting step is believed to be the diffusion of oxygen ions through the converted Na-.beta."-alumina material, since the only species that exhibits high mobility in Na-.beta."-alumina is the sodium ion. To convert alumina into Na-.beta."-alumina, both sodium and oxygen are required, and the diffusion of oxygen through Na-.beta."-alumina is very slow.
Materials analogous to Na-.beta.-alumina and Na-.beta."-alumina also have been found to be useful in various processes. For example, in "Potassium Beta"-Alumina Membranes" G. M. Grosbie and G. J. Tennenhouse: Journal of the American Ceramic Society, 65, 187 is disclosed membranes of the potassium analogs, K-.beta.-alumina and K-.beta."-alumina, that are made by ion-exchanging the sodium materials. These potassium materials have potassium-ion conductivity and may be used where the potassium-ion conductivity is required.
Sodium and potassium-.beta.-alumina and .beta."-alumina materials are also used in applications other than for battery applications, such as for sodium heat engines (SHE) or in general alkali-metal thermoelectric converters (AMTEC). Because of the continuing interest in these and the above battery technologies, a method for quickly producing electrolytes and other shapes without the disadvantages of sintered shapes would be an advance in the art.